356

ELEMENTS OF MATERIA MEDICA.

animals (as the crab and lobster) ; in the higher classes it is found inbone, hut the quantity of it is very small.

Preparation. —In the Dublin Pharmacopoeia, carbonate of lime(calcis carbonas pracipitatum, Ph . D.) is ordered to be prepared byadding a solution of carbonate of soda to a solution of chloride ofcalcium: double decomposition takes place, chloride of sodium is formedin solution, and carbonate of lime precipitated. Thus prepared, car-bonate of lime is directed to be used in the preparation of hydrargyrumcum creta. By some druggists it is employed, instead of prepared chalk,in the manufacture of aromatic confection.

Marble ( marmor , Ph. L., marmor album, Ph . D. & Ed.), or hard car-bonate of lime ( carbonas calcis durus, Ph . L.) is employed for the pro-duction of carbonic acid {vide p. 190); and, in some Pharmacopoeias , forthe preparation of chloride of calcium. For the latter purpose, espe-cially, white or statuary marble should be selected, on account of itsfreedom from iron.

Chalk {creta, Ph . L., creta alba, Ph. Dub. & Ed.) or friable carbonateof lime {calcis carbonas friabilis, Ph. L., carbonas calcis mollior, Ph . Ed.)is found in great abundance in the southern parts of England. Toreduce it to a fine state of division, and to deprive it of its coarser parts,it is submitted to the process of elutriation, and is then called preparedchalk {creta praparata, Ph . L. & Dub., carbonas calcis praparatus,Ph. Ed. & U. S. )

Carbonate of lime is prepared, for medical purposes, from severalmolluscous animals; as from the shell of the oyster {Ostrea edulis), thegastric concretions (called crabs' eyes or stones) of the crawfish {Astacusfluviatilis), and the crustaceous envelope of the claws of the crab {Cancerpagurus). The carbonate procured from these sources is called, respec-tively, testapraparata (Ph. L.), lapilli cancrorum, and chela cancrorum,and will be described in a subsequent part of this work {vide AnimalMateria Medico).

Properties. —Pure carbonate of lime is a tasteless, odourless solid.When heated to redness in a current of air its carbonic acid is expelled,leaving quicklime. It is almost insoluble in water ; one part of carbonaterequiring 1600 parts of water to dissolve it. It is much more soluble in car-bonic acid water: the solution reddens litmus, but changes the yellowcolour of turmeric paper to brown; and by boiling, or exposure to theair, gives out its carbonic acid, by which the carbonate of lime isdeposited.

Carbonate of lime is a dimorphous substance ; that is, it crystallizesin two distinct and incompatible series of forms,—viz. those of therhombohedric system (calcareous spar), and those of the right rectangularprismatic system (arragonite). According to Gustav Rose {Lond. fy Ed.Phil. Mag. June 1838), both calcareous spar and arragonite may beformed in the humid way, but tire first at a lower, the latter at a highertemperature: in the dry way, calcareous spar alone is formed. Bothminerals doubly refract the rays of light, and expand unequally in theirdifferent parts when heated; but calcareous spar has only one axis of nodouble refraction, whereas arragonite has two.